1. Field of the Invention
The present invention relates to a universal serial bus (USB) power supply, and more particularly to a universal USB power supply that is capable of charging any type of electronic device that is charged over a USB interface.
2. Description of Related Art
A universal serial bus (USB) interface is a commonly used connecting interface. A general USB interface has pin definitions comprising a VBUS terminal, a D+ terminal, a D− terminal and a ground terminal. The VBUS terminal and the ground terminal are used to output DC power if the USB interface connects to a DC power source. The D+ and D− terminals are used to transmit data. Compared with conventional computer connecting buses, the USB interface is capable of transmitting data and providing electric power.
Because of the above-mentioned features, some portable electronic devices are designed to have USB interfaces for connecting to computers and charging the portable electronic devices from the computers. Furthermore, a USB power supply is invented. The USB power supply is connected to the utility power or is connected to a cigar-lighter in a vehicle to obtain AC power or DC power and converts the AC power or DC power into DC powers with 5 volts. A portable electronic device can connect to and be charged by the USB power supply.
However, some manufacturers may want to restrict users to only charge the electronic devices with exclusive USB power supplies. With reference to FIG. 6, the electronic device has a USB interface (31), a power circuit (32), a switch (33) and a charge control unit (34). The power circuit (32) and the switch (33) are connected to the VBUS and ground terminals of the USB interface (31) to form a loop. The charge control unit (34) is connected to the D+ and D− terminals of the USB interface (31) and stores a default D+ voltage and a default D− voltage. For example, the default D+ voltage may be 3.5 volts and the default D− may be 4.1 volts. The electronic device's exclusive USB power supply (40) has a USB interface (41). The D+ and D− terminals of the USB interface (41) in the USB power supply (40) output signals having voltage levels identical to the default D+ and default D− voltages. When the electronic device (30) connects to the exclusive USB power supply (40), the charge control unit (34) in the electronic device (40) detects that the D+ and D− terminals of the USB interface (41) in the USB power supply (40) outputs the signals having voltage levels identical to the default D+ and D− voltages. The charge control unit (33) then controls the switch (33) to connect the loop of the power circuit (32), the VBUS and the ground terminals. Accordingly, the USB power supply (40) starts to charge the electronic device (30) so a current flows from the VBUS terminal of the USB power supply (40), passes through the power circuit (32) of the electronic device (30) and flows back to the ground through the ground terminal of the USB power supply (40). Otherwise, if the charge control unit (34) determines that the D+ and D− terminals of the USB power supply (40) outputs the signals having voltage levels different from the default D+ and D− voltages, the charge control unit (34) will control the switch (33) to cut the loop of the power circuit (32), the VBUS and the ground terminals. Therefore, the USB power supply (40) cannot charge the electronic device (30). Consequently, the electronic device (30) can only be charged by the exclusive USB power supply (40) because the default D+ and D− voltages are business secrets of the manufacturers. To charge the electronic device (30), the user has to buy the electronic device's exclusive USB power supply (40).
To overcome the shortcomings, the present invention provides a universal USB power supply that is capable of charging any type of electronic device to mitigate or obviate the aforementioned problems.